A black start is understood to mean that a power station is run up without any external energy supply. This is to say that there is no electrical energy available externally, for example for maintaining a local AC grid of the power station. A cause of this may be that an external AC grid to which the local AC grid is connected via a transformer, for example, has collapsed or that no energy can be drawn from such an external AC grid or that no external AC grid is present at all because the local AC grid is an island.
The term AC grid in this case primarily refers to the physical device comprising electrical connections, links, i.e. lines, and the like, but not to the AC voltage present at this device or provided by this device. A local AC grid is in this case understood to mean a limited AC grid. This may be an internal AC grid of a power station as long as the latter is disconnected from an external AC grid, but can also be a so-called island. The island may be either an autonomous island without any connection to an external AC grid or can equally also be a physically limited grid section which is disconnectable from a superordinate AC grid as long as said grid section is disconnected from the superordinate (external) AC grid. Often, the local AC grid does not comprise any additional loads which are not associated with the operation of the local AC grid, however, or these loads are switched off or disconnected during the black start.
The inverters of the power station which are connectable to the local AC grid are those with which electrical energy can be fed into the AC grid. In principle, these may also be bidirectional inverters with which electrical energy can also be drawn from the AC grid in order to be buffer-stored in a battery, for example.
In particular, the electrical energy which is fed by the inverters into the local AC grid originates from renewable energy sources such as wind turbines or generators and/or photovoltaic generators connected thereto, for example.
The term power station here means nothing more than the plurality of inverters which are connected to energy sources and with which electrical energy can be fed into the local AC grid. This may be a power station in the narrower meaning, for example a wind farm or a photovoltaic farm. The term power station also includes any other energy generation units having a plurality of inverters which are connected to any local AC grid, for example, to a grid section connected disconnectably to a superordinate AC grid, however.
During black-starting of a power station using a multiplicity of inverters connectable to a local AC grid, one problem is that the internal consumption of the local AC grid owing to transformer losses, cabling impedances, etc. which needs to be raised by the power station itself goes beyond the electrical energy which can be provided reliably by a single inverter. However, it would be very difficult to connect a plurality of inverters to the local AC grid at the same time, in particular as long as there is still no AC voltage present in the local AC grid.
DE 103 20 087 A1 discloses a method for operating a wind farm and a wind farm comprising a central apparatus for controlling the wind farm. In order to make the wind farm capable of a black start, said wind farm comprises a permanently excited wind turbine with motorless wind direction tracking. This permanently excited wind turbine can also be run up when no energy can be drawn externally. With the aid of the electrical energy from the permanently excited wind turbine, first at least one first wind turbine is run up which is not permanently excited and which is tracked in terms of the wind direction by a motor. This wind turbine builds up a rated frequency and rated voltage of a local AC grid again via a self-commutated inverter, with the wind turbines of the wind farm being connected to said local AC grid. The remaining wind turbines can be synchronized with the AC voltage in the local AC grid and can begin with the feed of power. This may be reactive power and/or real power. The remaining wind turbines can in this case cover the initial energy demand via the local AC grid. A precondition for the implementability of this known method is that the wind farm comprises a particular permanently excited wind turbine with motorless wind direction tracking and that the first wind turbine which is not permanently excited, with which the local AC grid is built up, provides sufficient electrical energy to cover the internal consumption of the local AC grid.
EP 1 993 184 A1 discloses a method for black-starting a wind farm and a wind farm, in which the internal consumption of a local AC grid is initially covered by a local energy source in the form of a large battery, an uninterruptible power supply or a generator, such as a diesel generator or a fuel cell, for example. The energy provided by this local energy source via the local AC grid is used to firstly start at least one first wind turbine. Then, successive further wind turbines are run up with the increasing energy available. In this case, the local power source must build up the local AC grid to which the individual wind turbines are connected and correspondingly cover the entire internal consumption of this grid. Furthermore, it needs to provide the electric power required for running up the first wind turbine. Correspondingly, this local power source which needs to be provided additionally must have large dimensions.
US 2012/0261917 A1 discloses a method for black-starting wind turbines, in which a diesel generator applies a preset AC voltage to the output of a wind turbine in order to imitate a rated AC voltage of a running local network. In this case, the real power and reactive power to be generated by the wind turbines on provision of the rated voltage is initially set to zero in order to enable connection without any sudden changes. The connection takes place stepwise wind turbine by wind turbine. In this case too, the diesel generator needs to be capable of covering the internal consumption of the local AC grid to which the wind turbines are connected.
Initial coverage of the internal consumption during a black start of a wind farm comprising a plurality of wind turbines comprising an energy store and subsequent stepwise connection of the wind turbines are also known from WO 2011/058170 A1.
In order to coordinate the operation of a plurality of voltage-setting inverters connected in parallel to an AC grid, i.e. of inverters which build up a preset voltage without direct communication between the inverters, the use of so-called frequency and voltage droops is known from EP 1 286 444 B1. The term frequency droop is in this case used to refer to a frequency/real power characteristic stored in the inverters which is used for controlling the frequency of the AC voltage output by the inverters depending on the real power output by the inverters: during voltage-setting operation of the inverters, the real power results from the present impedance. Depending on the output real power and stored frequency droop, the inverters set their frequency. The term “voltage droop”, on the other hand, is used to denote a voltage/reactive power characteristic stored in the inverters which is used for controlling the level or amplitude of the AC voltage output by the inverters depending on the reactive power output by the inverters: during voltage-setting operation of the inverters, the reactive power also results from the present impedance. In response to the output reactive power, the inverters set a voltage amplitude, which is corrected in relation to a reference value for the voltage setting, taking into consideration the stored voltage droop at the output.
In the case of current-setting inverters, i.e. inverters which output a preset current, the frequency droop likewise indicates the relationship between the output real power P and the frequency f of the AC voltage present at the output of the inverter. The voltage droop is correspondingly the relationship between the reactive power Q output by the inverter and the AC voltage present at its output. “AC voltage present at the output” is intended here and generally in this application to mean the magnitude of this AC voltage. The coordination of the operation of a plurality of current-setting inverters connected in parallel to an AC grid can also take place using frequency and voltage droops. Coordination of the operation with the aid of frequency and voltage droops is possible even in mixed groups of voltage-setting and current-setting inverters.
The document EP 2 632 012 A1 published after the priority date of the present patent application is concerned with the requirement that, in the case of a black start of a decentralized electrical energy supply grid, all of the connected grid formers find a common working point for building up a stable mains voltage. The operation of the grid formers in this case takes place with the aid of droop characteristics which compare an electric power called up by the electrical energy supply grid with the mains voltage. Each grid former determines, with the aid of its droop characteristics, its present voltage to be fed. A working point between a plurality of grid formers which is matched to one another in an electrical energy supply grid is achieved if all of the grid formers output an identical voltage to be fed. According to EP 2 632 012 A1, it would be most favorable to allow first a single grid former to preset the mains voltage and then to connect the remaining grid formers to the electrical energy supply grid. Islands are characterized by a low hierarchy, however. The grid formers can in this case only be connected jointly to the consumer loads. Owing to the connected consumers, the consumer load present during the black start can be too great for these individual grid formers, however, to generate the mains voltage. Therefore, the black start takes place with at least two coordinated grid formers together. This coordination could take place asynchronously, wherein each of the grid formers to be coordinated calculates the voltage to be fed into the grid on the basis of their dedicated power fed into the electrical energy supply grid via the droop characteristic. However, this is opposed by the knowledge that a common mains voltage cannot be set asynchronously if one of the coordinated grid formers cannot follow the other grid formers quickly enough on its droop characteristic because it has, for example, a current limit which it cannot exceed. This problem could be solved synchronously according to EP 2 632 012 A1 by virtue of a common master firmly presetting the setpoint values for the mains voltage. In this way, it will be possible to avoid a situation whereby excessively high currents are demanded of the grid formers owing to low temporary mains voltages. However, each individual grid former would need to be connected to the master, which can result in considerable costs, a high degree of susceptibility to faults and therefore to insufficient availability, precisely in the case of grid formers distributed physically over several kilometers.
In contrast, one concept of EP 2 632 012 A1 consists in moving the grid former stopped on its droop characteristic and to virtually give a thrust so that it can move on its droop characteristic again. This is achieved by virtue of the fact that, when the grid former has stopped on its droop characteristic, it is shifted onto a new point on the droop characteristic, from which it needs to output a current below its current limit for the power to be output. Thus, it can increase its power to be output to the electrical energy supply grid by means of the output current which is now variable again and can move freely on the droop characteristic. Therefore, EP 2 632 012 A1 discloses a method for synchronizing a feed-in voltage with a mains voltage of an electrical energy supply grid, wherein a property of the feed-in voltage is determinable on the basis of a droop characteristic, wherein the droop characteristic compares the property of the feed-in voltage with a feed-in power, wherein the feed-in power is consumed by the electrical energy supply grid when the feed-in voltage is applied, and wherein the property of the feed-in voltage is matched to a specific value when a limit for the feed-in power is reached. The properties of the feed-in voltage or the mains voltage are, for example, the frequency, phase angle and/or the rms value of the corresponding voltage. In the case of a black start of the energy supply grid, all of the grid formers involved are started synchronously by a switch-on signal and synchronized with one another via the electrical energy supply grid. In this case, it is sufficient if a master is connected to one of the grid formers, which master gives the start signal to said grid former. Even if, in accordance with EP 2 632 012 A1, it is intended to be possible for all of the grid formers to have an inverter as energy generation unit, which inverter is designed to convert electrical energy from a DC voltage source into a feed-in voltage, the grid former started by the master is specifically a diesel generator, which generates an initial voltage rise in the form of a voltage ramp. All of the other grid formers measure the voltage rise in order to become involved in the black start of the electrical energy supply grid, and the voltage rise is in this case used for starting the electrical power output of inverters which are connected as grid formers to the energy supply grid.
EP 2 190 110 B1 discloses a method for determining the loading capacity of a DC voltage source which is connectable to an AC grid with a preset line frequency via an inverter and a mains switch. In this method, a DC input voltage of the inverter, which may be the output voltage of the DC voltage source or a voltage in a DC link of the inverter, is detected when the mains switch is open and when there is actuation of the inverter which enables a four-quadrant operating mode. The inverter is actuated in such a way that it inverts the DC input voltage to a test frequency deviating from the line frequency. Deviating in this context means that the test frequency can be both lower and higher than the line frequency. It will often be higher than the line frequency. Given suitable selection of the test frequency in respect of the respective inverter and its use environment, a test load acting on the DC voltage source can thus be considerably increased above the internal consumption of the inverter at the line frequency. Specifically, it can be brought to a level which corresponds to the loading of the DC voltage source when said DC voltage source is connected to the grid, so that grid connection firstly takes place as early as possible, but secondly without any risk of renewed grid disconnection.